San Francisco 49ers quarterback Alex Smith and head coach Mike Singletary have a discussion on the sidelines, during the Philadelphia Eagles game, after Smith's fumble led to an Eagle score.

Photo: Brant Ward, The Chronicle

San Francisco 49ers quarterback Alex Smith and head coach Mike...

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San Francisco's Frank Gore on a 49-yard run in the first quarter against the Raiders on Saturday in Oakland.

Photo: Paul Chinn, The Chronicle

San Francisco's Frank Gore on a 49-yard run in the first quarter...

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Raiders Rolando McClain (55) chases 49er Delanie Walker (46), being tripped up by Raiders Kamerion Wimbley (96), as the Oakland Raiders take on the San Francisco 49ers in NFL pre-season action on Saturday August 28, 2010 in Oakland, Calif.

Oakland's 45-Marcel Reece with a 34 yard catch in the first quarter, being chased by 32- Michael Lewis and 38- Dashon Goldson, as the Oakland Raiders take on the San Francisco 49ers in NFL pre-season action on Saturday August 28, 2010 in Oakland, Calif.

The 49ers are 2-6 because they've lost four games by a field goal or less.

They've lost three games by a field goal or less because they had no quarterback who could save the day.

They have no quarterback who could save the day because 49ers coach Mike Singletary didn't want a Superman at quarterback, which he said isn't even the most important position anyway. He wanted a Clark Kent quarterback named Alex Smith.

Want to find the genesis of a promising season gone terribly wrong? Go back to the offseason, when Singletary crowned the unproven Smith as his one and only option at quarterback - then watched his unbending decision backfire in a flurry of interceptions, sack-fumbles and game-losing decisions.

Only now, with Smith tabled by a dislocated shoulder, is Singletary willing to try a Plan B after refusing all offseason to devise a credible backup plan at the quarterback position.

Can new starting quarterback Troy Smith still save the season? Singletary hopes so, because a lot of jobs are depending on the scout-team quarterback turned London starter to help the 49ers overcome the head start taken by the rest of the NFC West.

"All of the stuff that happened and transpired in the first half of the season, that's done," Singletary said. "We can't do anything about that, but we can learn from it."

So, what have the 49ers learned after starting 0-5 before winning two of three heading into the midseason bye?

Above all, that Singletary may not be the best judge of quarterbacks, which is critical because he's still the one doing the choosing.

He's the one who declared Shaun Hill the winner of the quarterback race in training camp last year, then benched him after a 3-2 start. He's the one who said Alex Smith was his unchallenged quarterback all offseason with no interest in bringing in a worthy backup.

Singletary said he did this because he wanted Smith to compete with himself and no one else. That's why the 49ers did not try to land a big-name quarterback - say, Donovan McNabb - in the offseason.

He gave no serious regard for the distinct possibility that Smith could get hurt again - saying their plan was for Smith to just not get hurt, and look at what happened in Week 7 at Carolina.

He left no room for the very real chance that Smith really isn't that good after all, that his career as a former No. 1 overall pick can't all be blamed on the constant change of offensive coordinators and playbooks.

Where has all this left Singletary and the 49ers? With a second-string quarterback, David Carr, he refuses to trust, and a third-string quarterback who's been asked to turn an entire season around.

Troy Smith did enough to beat the Broncos in his season debut Sunday. He will get a chance against the Rams next week to show he isn't a one-hit wonder.

Alex Smith will get another chance, even with a 1-6 record, if Troy Smith turns out to be third-string material, after all.

The decision all falls on Singletary, who has been so far, no good when it comes to picking his quarterbacks. This may be his last chance to get that right, because all fingers will point to him when this shakes out.

"Going into the second half of the season, guys, we got to go," Singletary said.